THE CIRCULATORY SYSTEM. 



105 



THE BLOOD. 



While, when physiologically considered, the blood is regarded, 

 with Bernard, best as an internal medium of exchange, histologically 

 it may be classed as a mesodermic tissue possessing a fluid inter- 

 cellular substance, the liquor sanguinis ; in the latter float the 

 cellular elements the blood-corpuscles. 



The morphological constituents of the blood are of two kinds, 

 the colorless or white corpuscles and the colored or red cells ; 

 to these must be added a third variety, the blood-platelets or 

 blood-plaques, which are probably constant and independent ele- 

 ments. 



THE COLORLESS CELLS OF THE BLOOD. 



The colorless blood-cells, or leucocytes, are not peculiar to 

 the blood, since they originate in lymphoid tissues and arc carried 

 by the lymphatic trunks into the blood-current, in which fluid they / 



FIG. 129. 



usually are observed. These cells represent 

 a widely-distributed element, whose names 

 are as various as are the localities in which 

 it is encountered. The "lymph-corpuscle," 

 ' ' lymphoid cell, " " adenoid cell, " " white 

 blood -cell," "leucocyte," " leucoblast, " 

 "wandering cell," etc., are but different 

 names for the same morphological element. 



The colorless blood-cell consists of a minute 

 nucleated mass of active protoplasm, when at 

 rest, presenting a round or spherical form and 

 measuring about 10 /* in diameter. In its 

 usual condition, however, the outline of the 

 corpuscle is undergoing continual variation, 

 these changes being known as amoeboid on 



account of their similarity to those exhibited by the amoeba. Under 

 moderate amplification the protoplasm of the leucocyte appears 

 faintly granular and includes a single nucleus, rarely multiple, which 

 is ordinarily somewhat obscured by the overlying cell-contents. 

 Additional coarse granules are of frequent occurrence, especially 

 within the protoplasm of particular cells ; these latter, sometimes 

 designated as the granular white corpuscles, are distinguished by 

 their large granules, small size, and affinity for eosin, and are re- 

 garded by some as a special variety of colorless blood-cell. The 

 exact nature and significance of such cells, however, are still un- 

 certain. Under high amplification the protoplasm of the white 

 blood-cell often displays an imperfect reticulation as a transient 



Colorless blood-cells of man, 

 highly magnified : r, cor- 

 puscle in condition of rest, 

 as a spherical mass of proto- 

 plasm ; the other cells are 

 actively moving and exhibit 

 a hyaline apparently struct- 

 ureless substance in the most 

 advanced parts of the cells. 



